The industrial significance of mercaptans or thiols means that many studies have been carried out for the purpose of perfecting the preparation of these compounds.
In particular a process is known which is widely employed and which implements the reaction of hydrogen sulphide with an alcohol or an olefin. In such a reaction a by-product which is obtained in particular comprises one or more thioethers, which result from secondary reactions and, primarily, from the reaction of the mercaptan (formed in the main reaction) with the starting reactant, in other words either the alcohol or the olefin, depending on the process used.
Thioethers obtained as by-products during the preparation of mercaptans are not generally of commercial significance.
It is therefore essential to improve the selectivity of the catalyst employed in the reaction, so as to increase the yield of thiol, more particularly when the latter is obtained by addition of hydrogen sulphide to an olefin, a reaction often called sulfhydration.
Methods of sulfhydration have been proposed which are directed at reacting hydrogen sulphide (H2S) with an olefin under pressure in the presence of various catalysts.
Many catalysts have been proposed in the prior art, and in particular supported phosphoric acid (by U.S. Pat. No. 2,950,324), silica with small amounts of alumina (by U.S. Pat. No. 2,951,875), a synthetic zeolite (by U.S. Pat. Nos. 4,102,931 and 5,453,544) or an ion exchange resin (cf. U.S. Pat. No. 4,102,931). The ion exchange resins allow values to be obtained that are of interest for the conversion of the olefin and the selectivity to mercaptan. However, these resins undergo degradation at 100° C. and above, and are completely decomposed at 140° C. The result of this is that they do not allow the catalysis of sulfhydration reactions, which, owing to the olefin used, require a high temperature.
U.S. Pat. No. 6,162,952 describes a catalyst supported on an oxide (TiO2 or ZrO2) associated with an acidic H2SO4, (NH4)2SO4 or WO3 site. However, owing to the likewise-limited stability of these solids at temperature (at approximately 200° C.), this catalyst has the same drawback as before.
U.S. Pat. No. 3,036,133 describes the preparation of ethyl mercaptan and diethyl sulphide by addition of H2S to ethylene in the presence of a catalyst comprising silica or alumina activated with a heteropolyacid, or one of its alkali metal or alkaline-earth metal salts. However, this catalyst, when employed for an olefin other than ethylene, such as propene or butene for example, produces a low conversion to olefin. Moreover, the selectivity to mercaptan, and hence the yield of mercaptan, are generally insufficient.
A solid catalyst comprising a 12-phosphotungstic acid supported on silica is also described by U.S. Pat. No. 5,420,092. That document teaches, more generally, the use of a heteropolyacid in combination with a metal from group VIII, but in the distant field of the isomerization of paraffins.